The Stirling engine is a piston engine in which the working gas is not vented after each cycle, but instead permanently contained within the cylinder. The working gas is usually air, helium, or hydrogen. The engine works by exposing the working gas to an external heat source, and then to a cold source which is colder than the external heat source. The gas expands when exposed to the heat source and contracts when exposed to the cold source. The engine has two pistons which extract useable work from the expansion of the gas and also serve to move the gas from the heat source to the cold source. With proper design, the Stirling engine can extract work from the gas both on the heating cycle, and on the cooling cycle. Stirling engines can have efficiencies up to 50% of the Carnot efficiency based upon the temperature difference between the heat source and the cold source.
There are three types of Stirling engines. The alpha Stirling engine uses two power pistons which operate in separate cylinders. The gas in the hot cylinder expands driving a piston which imparts energy to a flywheel. The flywheel turns and the piston in the hot cylinder forces the gas through a pipe where it enters the cold cylinder and is cooled. The gas is allowed to expand in the cool cylinder imparting further power to the flywheel. The piston in the cool cylinder then moves to compress the gas and drive it back through the pipe to the hot cylinder where it is heated, expands and begins the cycle again. The beta Stirling engine has two pistons operating in a single cylinder. The cylinder is divided into a hot region and a cold region. One piston is a power piston which is acted upon by the expanding gas in the hot temperature region and also compresses the gas in the cold region. This piston imparts power to the fly wheel. A displacer piston moves the gas between hot and the cold regions. A gamma Stirling engine is similar to a beta Stirling engine in that it has a power piston and a displacer piston. The two pistons operate off the same flywheel. The gamma Stirling engine has two separate but freely communicating cylinders with the power piston in one cylinder, and the displacer piston in the other cylinder.
The Stirling engine has several advantages over internal combustion engines. The Stirling engine is not limited in the type of fuel it can burn, or even limited to burning of fuel to create heat to operate the engine. Any heat source may be used to provide heat to the hot area of a Stirling engine. The working gas of a Sterling engine is not a hot combustion gas, as in an internal combustion engine. Thus, the Stirling engine is easier to lubricate than an internal combustion engine. The Stirling engine also has higher efficiency than an internal combustion engine. Finally, the Stirling engine runs quietly. In spite of these advantages, the Stirling engine is used only in specialized applications such as aboard submarines.
The Stirling engine has some major disadvantages which prevent its wide spread use. The Stirling engine is larger than an internal combustion engine for the same output. In addition, the cost of a Stirling engine per kilowatt is higher than that of the less efficient internal combustion engine. Accordingly, except for specialized applications, the Stirling engine is not widely used. There is a need to increase the efficiency of the Stirling engine in order to achieve the advantages that it offers.